US Bridges

SR 99 over Duwamish River

Fair

King County, Washington · 0.1 N JCT SR 509 · Structure no. 0004872A0000000

Built
1956
69 years old, rebuilt 1998
Condition
Fair
inspected September 2024
Type
Movable - Bascule
Steel
Length
3,010 ft
1 main span
Daily traffic
48,204
2023 count
Owner
State
State Highway Agency

About this bridge

SR 99 over Duwamish River is a steel movable - bascule carrying SR 99 over Duwamish River in King County, Washington. It was built in 1956 and reconstructed in 1998, so it is 69 years old, older than the Washington average of 49.8 years.

The most recent inspection reported to the Federal Highway Administration rates it in fair condition with a lowest component rating of 5 out of 9. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

It carries about 48,204 vehicles a day, roughly 6% of them trucks, projected to reach 72,459 by 2039. The structure is 3,010 ft (917.4 m) long and 58 ft wide. If it were closed, the detour would be 5 mi (8 km).

Location

Open in Google Maps
CarriesSR 99
CrossesDuwamish River
Location0.1 N JCT SR 509
RouteState Highway 99
CountyKing County
StateWashington
Coordinates47.53758, -122.33469
Functional classUrban Principal Arterial - Other Freeway or Expressway

Condition and inspection

Fair

The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

ComponentRatingMeaning
Deck5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Superstructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
Channel and channel protection8 Very GoodNo problems noted.
Last inspectionSeptember 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation5: Somewhat better than minimum adequacy to tolerate being left in place as is
Deck geometry4: Meets minimum tolerable limits to be left in place as is
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating46 US tons
Inventory rating28 US tons

Structure

Structure typeMovable - Bascule
MaterialSteel
DeckOpen grating
Main spans1 (+23 approach)
Design loadHS 20 + Mod
Structure length3,010 ft (917.4 m)
Longest span277 ft (84.4 m)
Deck width58 ft (17.7 m)
Roadway width44 ft (13.4 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic48,204 (2023)
Projected traffic72,459 by 2039
Trucks6%
Lanes on structure3
Traffic directionOne-way traffic
CarriesHighway and pedestrian
CrossesHighway, waterway and railroad
Detour if closed5 mi (8 km)
TollOn a free road
National Highway SystemYes

Ownership and planned work

OwnerState Highway Agency
Maintained byState Highway Agency
Historic statusListed on the National Register of Historic Places
Work proposedRehabilitation because of general structure deterioration or inadequate strength
Estimated improvement cost$87.2 million (2022 estimate)
NavigationNavigation control on waterway (bridge permit required)

Frequently asked questions

When was SR 99 over Duwamish River built?

It was built in 1956, making it 69 years old in 2025. It was reconstructed in 1998.

What condition is SR 99 over Duwamish River in?

The National Bridge Inventory rates it in fair condition. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound. The last reported inspection was in September 2024.

How much traffic does SR 99 over Duwamish River carry?

About 48,204 vehicles per day (2023 count), of which roughly 6% are trucks.

How long is SR 99 over Duwamish River?

The structure is 3,010 ft (917.4 m) long with a longest span of 277 ft (84.4 m) and a deck 58 ft wide.

Who owns SR 99 over Duwamish River?

It is owned by the State Highway Agency.